Left foreleg (left arm) electrode placed just below the point of the elbow on the back of the left forearm – right foreleg (right arm) electrode placed just below the point of the elbow on the back of the right forearm

Lead II: LL–LA

Left hindleg (left leg) electrode placed on the loose skin at the left stifle in the region of the patella – left foreleg (left arm) electrode placed just below the point of the elbow on the back of the left forearm

Lead III: LL–RA

Left hindleg (left leg) electrode placed on the loose skin at the left stifle in the region of the patella – right foreleg (right arm) electrode placed just below the point of the elbow on the back of the right forearm

aVr: RA–CT

Right foreleg (right arm) electrode placed just below the point of the elbow on the back of the right forearm – the electrical centre of the heart or central terminal × 3/2

aVl: LA–CT

Left foreleg (left arm) electrode placed just below the point of the elbow on the back of the left forearm – the electrical centre of the heart or central terminal × 3/2

aVf: LL-CT

Left hindleg (left leg) electrode placed on the loose skin at the left stifle in the region of the patella – the electrical centre of the heart or central terminal × 3/2

CV6LL: Vl–CT

Vl electrode placed in the 6th intercostal space on the left side of the thorax along a line parallel to the level of the point of the elbow – the electrical centre of the heart (central terminal)

CV6LU: V2–CT

V2 electrode placed in the 6th intercostal space on the left side of the thorax along a line parallel to the level of the point of the shoulder – the electrical centre of the heart (central terminal)

V10: V3–CT

V3 electrode placed over the dorsal thoracic spine of T7 at the withers – electrical centre of the heart. The dorsal spine of T7 is located on a line encircling the chest in the 6th intercostal space (central terminal)

CV6RL: V4–CT

V4 electrode placed in the 6th intercostal space on the right side of the thorax along a line parallel to the level of the point of the elbow – the electrical centre of the heart (central terminal)

CV6RL: V5–CT

V5 electrode placed in the 6th intercostal space on the right side of the thorax along a line parallel to the level of the point of the shoulder – the electrical centre of the heart (central terminal)

Base-apex: LA–RA

Left foreleg (left arm) electrode placed in the 6th intercostal space on the left side of the thorax along a line parallel to the level of the point of the elbow – right foreleg (right arm) electrode placed on the top of the right scapular spine

Complete heart block is rare in horses and is usually associated with inflammatory or degenerative changes in the AV node,23 although it may occur with electrolyte imbalances or other metabolic abnormalities such as is seen with foals with uroperitoneum, particularly when anaesthetized.24,25 Complete heart block has also been observed as a congenital defect26 and associated with mediastinal lymphoma27 and rattlesnake envenomation.28 Horses with third-degree AV block will typically have severe exercise intolerance and frequent syncope. The resting heart rate (ventricular rate) is very slow (usually ≤20 bpm) but regular with a more rapid, independent atrial rate. The heart rate does not increase appropriately with a decrease in vagal tone or an increase in sympathetic tone. The S1 and S2 are usually loud and regularly spaced with more rapid independent S4 (usually ≤60/minute), which are also regularly spaced. Occasional “bruit de cannon” sounds, caused by the summation of S4 with another heart sound (S1, S2 or S3) are detected. An irregular rhythm may be detected in some horses with concurrent paroxysms of ventricular tachycardia.

Prolonged periods of SA arrest, profound sinus bradycardia or high-grade SA block may be indicative of sinus node disease, termed sick sinus syndrome.30 Ventricular escape rhythms may occur during prolonged pauses. Sinus node disease is rare in horses but inflammatory and degenerative changes must be considered possible aetiologies. Affected horses may have a history of collapse or weakness. These horses should be carefully evaluated with exercising electrocardiography and the response of the horse to vagolytic and sympathomimetic drugs determined. It may be possible to increase the heart rate and abolish the dysrhythmia during exercise tests, although the maximal heart rate may be reduced and the dysrhythmia may recur shortly after cessation of exercise.30 Corticosteroids should be initiated for horses with life-threatening abnormalities of sinus rhythm, in the hope that pacemaker implantation will not be necessary. Definitive treatment of sick sinus syndrome is pacemaker implantation30 (see Chapter 14).

SVPD originate in the atria before SA nodal discharge.21 There is often an underlying regular rhythm and the SVPD may or may not be conducted to the ventricles. On auscultation, beats occurring earlier than normal are detected. Electrocardiography reveals a normally configured QRS–T complex, occurring prematurely. A bizarre P wave may be visible or may be hidden in the preceding T wave (Figs. 13.5 and 13.6). Supraventricular or atrial tachycardia is defined as more than four SVPD occurring in sequence.21 Horses with frequent SVPD or supraventricular tachycardia are often able to maintain a normal ventricular response rate as some of the SVPD are blocked at the level of the AV node (see Fig. 13.6).

Atrial fibrillation (AF) is a common arrhythmia in horses.35–38 The estimated minimum frequency (i.e. no. of episodes per race starts) of AF in Japanese racehorses is 0.03% with an estimated minimum prevalence (i.e. no. of horses with AF per no. of racehorses) of AF among Japanese racehorses being 0.29%.38 A similar frequency has been reported in the UK.39 In the vast majority of horses that develop AF during racing the dysrhythmia is paroxysmal.38,39 There do not appear to be any gender predispositions but AF is more prevalent in racehorses >4 years of age compared to 2-year-olds. Horses racing on turf were more likely to develop AF than those racing on dirt in one study.38 AF is also very common in large draft breeds40 and Standardbreds have been over-represented compared to hospital populations in some studies.35–37